Interpretive Summary: In beef cows, GnRH-induced ovulation of small follicles decreased pregnancy rates and increased late embryonic mortality. Furthermore, small ovulatory follicle size was associated with reduced circulating preovulatory concentrations of estradiol. Beneficial effects of estradiol for pregnancy in cattle could be through improved oocyte maturation and quality, improved sperm transport and viability, or through improved environment of the uterus for pregnancy. Cows that ovulate a smaller follicle after an injection of gonadotropin releasing hormone have a reduced chance of becoming pregnant. This results from both reduced pregnancy establishment and greater embryonic death. We assumed ovulation of an immature ovum or from an immature follicle to be the root cause. However, we have also learned that size of the ovulatory follicle is correlated with blood concentration of estradiol. We conducted two experiments to study the effects of estradiol on pregnancy. In the first study, we measured estradiol in blood of cows at the time of GnRH2 injection (7 days before recovering their embryo). We also measured estradiol in blood of cows at GnRH2 (7 days before giving them an embryo). Pregnancy was determined 21 and 65 d after cows received an embryo. Circulating estradiol was positively correlated with follicle size at GnRH2 and progesterone at embryo transfer. Donor cows with greater estradiol at GnRH2 were more likely to yield an embryo than an unfertilized oocyte. Donor and recipient cows were classified into four groups based on high or low circulating estradiol concentration. Pregnancy rate at d 27 were low-low (45%), low-high (65%), high-low (43%), and high-high (61%) groups. Because pregnancy rates were greater when embryo recipient cows had greater estradiol, a second study was conducted. In the second study, cows were given estradiol or no treatment one day before GnRH2 and size of their follicle was measured. Compared with the control cows, estradiol treated cows with small follicles had greater pregnancy rates. In summary, decreased estradiol at induced ovulation decreased fertilization rate and pregnancy rate in cows. Providing estradiol before ovulation improved pregnancy rate in cows induced to ovulate small follicles.